What if China never turned itself to isolation and let Zheng He, and Hong Bao to explore more until they landed in a fertile area let say in Botany Bay or in Darwin and decides to colonize Australia after the Ming is being overthrown by the Manchus.
or
Dutch explores the Australia's east coast and lands in Botany Bay and colonize then the Dutch realize themselves that their numbers are too small to colonize in their own right and decides to recruit Chinese or Indonesian workers to develop Australia in case they discover a gold deposits or cultivates the arable land.
What would be the effects to world history if Australia is largely populated by the Chinese and Malays instead of the Europeans in OTL?
You've proposed two very diffferent scenarios and will manifest themselves in different but not entirely dissimilar ways.
The former case, Australia perhaps start out as an anchorage for the Ming fleet, and then a refugee during the Ming collapse. This would resemble the relocation of the royal court to Burma in OTL, except they likely wont be pursued and hunted down given the distance. It does mean that the literate classes who still believe in a Han self ruled state would go to Australia. It wont be just a one time thing, but there would be a continous emmigration during the Qing dynasty, whatever official disapproval of emmigration. The dynasty itself may not last. But ultimately a government resembling a Chinese dynasty would exist, and in some way rival the Qing Empire for legitimacy.
The second case would mean an Australia as part of the Dutch Empire with a large Chinese population doing and running daily business for their Dutch overlords. Perhaps the Chinese would be so dominant that the Dutch would live with anxiety that their role is marginalized. That would be a close analogy to the Dutch colony at Jakarta. However this will not be a Chinese colony in all but name as the Dutch feared. Because what actually happened was the Dutch created an aristocratic class out of the wealthy Chinese business class to lord over the fresh off the boat Chinese immigrants. This divide and rule strategy created a bifurcated Chinese society where the bottom remained very Chinese and the top became "more Dutch than the Dutch".
It is possible that at some time this colony would seek independence. The result would be a state with hybrid cultural characteristics, which would interact with Holland and China in interesting ways. Perhaps it would offer a model for modernization of China, as Hollandized-Chinese joint stock companies seek trade expansion to their ancestoral homelands.
How a remanent Ming successor in Australia deal with the outside world would also be interesting. Cut off and stranded in Australia, the Chinese colonies would be forced to seek maritime trade to survive. It's reaction to the later Portugese and Dutch traders would likely be more business minded than the autarkic Qing dynasty. So this colony, too would be more open to the outside world. But it would also be a closer reflection of China than our other case. In many ways, it could be what America was to Britain. It's cultural ties to the ancestoral land would compell it to interject itself into Qing politics in some way. Everytime a rebellion break out in China, the Manchus would have to be mindful of a possible overseas connection.